Hot gas apparatus including a regenerator



Aug. 14, 1951 F. K. DU PRE 2,564,100

HOT GAS APPARATUS INCLUDING A REGENERATOR Filed Aug. 7, 1947 Fla. 1 2

FIG. 3 F164 LNVENTOR ATTORNEY Patented Aug. 14, 1951 HOT GAS APPARATUS INCLUDING A REGENER'ATOR Frits Karel du Pr, Tarrytown, N. Y., assignor to Hartford National Bank & Trust 00., Hartford,

Conn, as trustee Application August '7, 1947, Serial No. 767,305

'20 Claims.

I My invention relates to hot-gas apparatus of the type which utilizes a regenerator through which a working medium flows in an alternate manner.

This application is a continuation-in-part of my copending application Ser. No. 674,492 filed June 5, 1946, now abandoned.

As my invention is particularly useful in hotgas external combustion engines, I shall describe the same in this connection. However, my invention is not limited to such engines but is applicable to other types of hot-gas apparatus such as a refrigeration machine operating on the reverse principle of the hot-gas cycle and utilizing a regenerator which alternately removes and replaces thermal energy contained in a working medium in the engine as the medium. moves through a predetermined cycle.

Although hot-gas apparatus has been known for many years and has had a very limited commercial success, such apparatus has to a large extent gone out of use in recent years. This has been due mainly to the fact that such apparatus, as compared for instance to internal combustion engines, had a large weight per horsepower, had

though there has been some suggestions as toimprovin the general design of said engines, as far as I am aware such suggestions have not lead to the production of a commercially successful engine.

The main object of my invention is to increase the thermal efficiency of hot-gas apparatus.

v A further objectof my invention is to increase the operatingspeedo'f hot-gas apparatus.

Another object of my invention is to increase the horsepower output per pound of engine weight of such hot-gas apparatus.

Still another object of my invention is to .provide a hot-gas apparatus capable of operating at an increased temperature difference between the heated and the cooled end of th apparatus by virtue of an improved regenerator.

These and further objects of the invention will become apparent as the specification progresses.

The hot-gas apparatus according to the invention comprises a hot chamber connected to a cold regenerator space and has "a surface area 'per'unit volume of the space occupied between about 25 cmfilcm. to 750 cmF/cmfi, preferably between about cmf /cm. and'200 cm. /cm. Furthermore the mass of filling material has a thickness in the direction of flow of the working medium between about 0.3 cm. and 8 cm., preferably between about 0.8 cm. and 4 cm. in the case of a hot-gas engine and preferably between about 5 cm. and 8 cm. in the case of a refrigerating machine operating onthe reverse principle of the hot-gas cycle, a heat capacity between about three to twenty-five times the heat capacity of the working medium flowing through the regenerator during one stroke of the engine and a total volume of the voids less than about five times, preferably less than about three times the mean volume of the working medium passin through the regenerator in one direction during each cycle. In addition the regenerator has a space factor between about 3 and 60, preferably about 6 to 25. The filling material furthermore has a low overall heat conductivity. By the expression heat capacity I mean the. quantity of heat needed to increase the temperature of the filling mass 1 centigrade. The expression space factor means the ratio between the overall volume occupied by the regenerator to the volume occupied by the material of the filling mass of the regenerator.

In constructing a regenerator having values within the ranges as outlined above, it will not in general be feasible to utilize a combination of the extremes of the values listed in order to obtain the best results. For example, if it is found desirable to use a maximum value of surface area per unit volume of space occupied, then intermediat values of regenerator thickness, and/0r heat capacity, will generally serve best .in the combination. If such a procedure is not followed in the construction of the regenerator, the operating elficiency of the regenerator although good, may not be as high as when using the proper combination of the values within the ranges specified.

In one embodiment of my invention I use as the filling material, metallic filamentary material, the average cross-sectional area of the filaments being about l 10- cm. to 300 l0 cm. preferably 3 X lO-cm? to 40 10- cm?.

In another embodiment of my invention I utilize a filling material comprised *of glass, asbestos or cotton fibers which are sandwiched between two layers of porous material which serv to keep 5:3 the fibers in their given location.

By way of illustration, a form of my invention is shown in the attached drawing in which:

Fig. l is a longitudinal section view of a hotgas apparatus in accordance with the invention.

Fig. 2 is a partial schematic sectional drawing illustrating the position of a regenerator in a hot-gas apparatus.

Fig. 3 is a cross sectional view of a regenerator.

Fig. 4 is another cross sectional view of a regenerator.

Referring to Fig. 1 of the drawing the numeral I indicates an enclosed chamber having heat insulating walls, this chamber serving as the heating chamber when the apparatus is used as a hot-gas engine, or serving as the cooling chamber when the apparatus functions as a refrigerator and operates on the reverse principle of the hot-gas cycle. A displacer II and a piston I3 operate on a working medium, such as air, hydrogen, helium, nitrogen and the like, within the engine and are each connected to a crankshaft I2, in such a manner, that changes in the volume of the space above the displacer and of the space above the piston are displaced about 90 relative to each other. As can be seen from this structural arrangement the working medium alternately flows between the space above the displacer and the space above the piston in a closed path and must pass through a regenerator I4 which is located in this path. The space between the top of the displacer I I and the regenerator I4 is at a high temperature when the apparatus is used as a hot-gas engine and for present purposes I will designate this space as the hot chamber, while the space between the regenerator I4 and the top of the piston I3 is at a lower temperature and for present purposes I will designate this space as the cold chamber. To prevent heat losses due to a conduction of heat through the housing of the regenerator and the filling mass itself it is advisable that both of them consist of a material with low heat conductive properties. A crankcase I5 is provided, which crankcase is generally made fluid tight and contains lubricating oil, and furthermore serves as a reservoir for the working medium of the cycle, the working medium being usually introduced into the cycle through a conduit and valve system (not shown) which interconnects the crankcase with the space above the piston.

When the hot-gas apparatus shown in Fig. 1

is used as a hot-gas engine, the power which is developed is delivered by the crankshaft I2, whereas, when the said apparatus is used as a refrigerator, power is applied to the crankshaft by means of a prime mover, for example, a motor (not shown).

Fig. 2 illustrates the position of the regenerator shown as I6 relative to the hot chamber shown as I! and the cold chamber shown as I8. It is to be noted that the cross sectional area of the regenerator is preferably made equal to that of the passageway interconnecting the hot and cold chambers and is positioned so that all of the working medium flowing between the respective chambers must flow through the regenerator.

A regenerator I9 according to a preferred embodiment of my invention is shown in Fig. 3. In this instance the regenerator mass comprises uniformly-distributed filling material, such as fine metallic wire of a ferro-chromium alloy or of a 'ferro-nickel-chromium alloy. I have found that if the ratio of the surface area of the regenerator filling to its volume is in the range of about 25 cmfi /cm. to about 750 cmP/cm. preferably between about 100 cmF/cm. and 200 cmF/cmfi, and if the individual filaments have a cross sectional area within the range of about 1 l0- cm. to about 300 l0* cm. preferably about 3x10 cm?v to 40 1O cm. the rate of heat transfer is directly proportional to the flow velocity of the medium, a fact which is quite unexpected in view of the literature concerned with heat phenomenon, and has the advantage that the regenerator efliciency will be practically independent of engine speed. Furthermore, I have discovered that the regenerator filling material must be uniformly distributed, whether the pattern be regular or random, if the regenerator is to provide satisfactory operation. By uniformly distributed it is meant that each unit of the regenerator filling mass, regardless of location within the regeneratonwill be comparable with respect to average density distribution.

The regenerator mass in the preferred embodiment of my invention as shown by Fig. 3 has a heat capacity of about five times that of the heat capacity of the working medium passing through the regenerator in one direction during each cycle. Quite unexpectedly I have found that it is possible to use a factor as low as three times instead of the above mentioned preferred factor, and still have the regenerator operate at a high efficiency. Likewise I have found that the total volume of the voids in the regenerator must be less than about five times and preferably less than about three times, the piston displacement of the apparatus. Also the regenerator has a space factor between about 3 and 60, preferably between about 6 to 25.

The thickness of the regenerator mass in the direction of flow of the medium must, as previously pointed out, be within the range of about 0.3 cm. minimum to about 8 cm. maximum preferably between about 0.8 cm. and 4 cm. If the minimum thickness is less than the said minimum, then regardless of the cross sectional area of the regenerator, an insuflicient transfer of heat between the regenerator and the working medium will take place, and a high speed engine of high operating efliciency would not be realized. When the regenerator has a thickness greater than the above stated maximum, frictional losses of the working medium similarly reduce the efliciency of the hot-gas apparatus and preclude the attainment of a high speed hot-gas apparatus. I have found that in order to produce a satisfac tory high speed hot-gas apparatus the maximum permissible power loss through the regenerator must be less than about 20%, preferably less than 10%.

The efficiency of a regenerator is understood to mean the relative ability of the regenerator to change the temperature of the flow medium to a temperature corresponding to the temperature of that end of the apparatus toward which the medium is flowing. The regenerator shown by Fig. 3 which embodies the principles of our invention, exhibits an operating efficiency of about and better. With regenerators having such a high efliciency and incorporating the principles of our invention, hot-gas apparatus can be made to operate at a high thermal efficiency and at speeds of 2000 R. P. M. and higher.

In Fig. 4 another form of a regenerator is illustrated. The number 20 indicates a filling material made from substances such as glass fiber, asbestos fiber, or other finely-divided materials which Withstand the operating temperatures used. "when the apparatus-shown in Fig. 1 is used as a refrigerator, the filling may :also "consist of cotton vvaddiug. 'nr-iei rineiples of iny inven'tion are applied ih "he fiesi'g n of "such a 'regen'erato'r mass, that is, :"the surface area (per unit volume or "the space occupied is between --about eiri.- /em. to "750 ems/"cum, an average cross sectional-area of in'dividuarniaments being about '1' 10-" cm.' to 300 I0= etc. With such a form "of filling material I have overcome "the disadvaritagescaused'by the tendency orithe "materialto break into small'parti-cles to their rather brittle nature utilizing such layers rt- '21 also servingtoretain the mass in its originalposition arid slia'pe. A :re'genen ator-su'ch as shown Fig. 4 is particularly usenil when the hot-gas 'ap at s or the invention is used as a reirigerat 'v'vhereinthe temperatures of the working medium are at a relatively lowl'evel.

While I have shown "and described "particular embodiments of my in'v on i do not'wish to be limited thereto since variations may "'ade in form without departing iron: the true spirit "of the invention.

Wha't I claim is:

f "1. In a hotas ap aratus o erating with a given amountof new medium 'and'ha'vingcylinder means defining a'h'ot chamber anda-eoldehamher, "a housing having an inlet opening and an outlet opening interconnedting said chambers; a heat regener'ator comprising a mass of finely divided filling materiaieorriprising elements having a cross sectional area Within the range of about 1 1 0 emf to about moxieam}, said mass having a ratio or surfae fare fto voiumejwithin "the range of about 25 emf/cm. to 750 w i /ems, being about 0.3 em. minimum to about 8 em. maximum in thickness, having a {total heat ca- 'pacity greater than about three times the heat capacity of the flow medium passingin one direction through said regenerator, and having a total volume of voids less about five times that of the mean volui n'eof the new meuium' 'assing in one direction throughsaid'regeneratqr.

2. "In a hot-gas apparatus operating with a given amount or flow medium and comprising cylinder means defining a hot cham er and a cold chamber, a housing having an inlet opening "andan Outlet opening "interconnecting 's'ai'd chainbers; a heat regenerator comprising a inass of finely divided filling material eo'inprisingeleme'nts Having a cross sectional area within the range 'of about 3X10;6 (:in. to about 40x10 iz'fnfi,

said mass having a ratio of surface area tqvoiu 'ie Within the range of about em icm to 200 cmi /emfi, being about 0.8 imam to about *4 cm. maximu'min thickness, having a total heat capacity greater than about five times the heat capacit or the how medium pas rig in one 'd-iree'tion through said regenerator, and having a total volume or voids than about three times that of'the meanvolume or the flow medium passing in one diretion through said regenera ion 3. In a hot-gas apparatus operating "a given amount of new medium and having cylinder means defining a not chamber and a cold chainbar, a housing having an inlet opening and an:

outlet opening interconnectin said chamber-s; a heat 'regenerator com rising a mass or nneiy divided fining material comprising "elements havin a cross sectional area Within tn range of about 1x 1o= to about sooeziin said massive having a :ratio Ofsurface area to volume fvvithin the range of about2-5 Cm. /Cm. t0 750 cms /cmi i, being about 0.3 'cm. minimum to about -8 cm. maximum in thickness, having a total heat cazpaoity greater than about three times the heat capacity of the flow medium passing in one dl rection tl iroug-h said rege'nerator, and having a total volume of voids less than about five times 'th'atof the mean volume of the flow medium passing one direction through said regeneraton said filling material consisting of a ferro-alloyselected from the group consisting of tetra-chromium and ferro-nickel-chromium.

4. :In a hot-gas apparatus operating with a given amount of flow medium and having cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat 'regener-ator comprising a mass of finely dividedfilling material comprising elements having a cross-sectional area within the range of about e1 *10- cm. to about 300x10 cm? said mass having a ratio of surface area to volume within the range of about 25 cmP/cmfi to 750 cmfi/cmf i, beingabout 0.3 cm. minimum to about 8 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the flow medium passing in one direction through said regenerator, and having a total volume of voids less than about five times that of the mean volume of the flow medium passing in one direction through said regenerator, said filler consisting of a material of. low heat conductivity selected from the group consisting of glass, asbestos and cotton fibers.

5. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regen'erator comprising a mass of finely divided filling material comprising elements having a cross sectional area within the range of about 1 10' cm. to about 300x10 cmfi, said mass having a ratio of surface area 'to volume within the rangeof about 100 cinF/cin. to 200 cm. /cm. being about 0.3 cm. minimum to about 8 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the flow medium passing in one direction through said regener'ator and having 'a total volume of voids less than about five times that of the mean volume of the flow medium passing in one direction through said regenerator.

6. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet op'enmg and an Outlet opening interconnecting said chambers; a heat regenera'tor comprising a mass or then divided filling material comprising e1eiii'ents a tress sectional area Within the range or about 1X10 cm; to about 300 10= omi said mass having a ratio o'f's'urface area to volume Within the range of about cin-.'*' /cir'1; to 750 ci'n; /cm-. being about 0.8 cm. "minimum to about 4 'cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the now medium passing in one direction through said re'generator and having a total volume of voids less than about five times that 'of the mean volume of the flowmedium passing in one direction through said regenerator.

V. In a hot-gas apparatus operating with a given amount of flowmedium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material comprising elements having a cross sectional area within the range of about 1X10 cm." to about 300 cmfi, said mass having a ratio of surface area to volume within the range of about cmF/cm. to 750 cmfi/cmfi, being about 0.3 cm. minimum to about 8 cm. maximum in thickness, having a total heat capacity greater than about five times the heat capacity of the fiow medium passing in one direction through said regenerator and having a total volume of voids less than about five times that of the mean volume of the flow medium passing in one direction through said regenerator.

8. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers: a heat regenerator comprising a mass of finely divided filling material comprising elements having a cross sectional area within the range of about 1 10 cm. to about 300x10- cmfi, said mass having a ratio of surface area to volume within the range of about 25 cm. /cm. to about '750 cm. /cm. being about 0.3 cm. minimum to about 8 cm. maximum in thickness, havinga total heat capacity greater than about three times the heat capacity of the flow medium passing in one direction through said regenerator and having a total volume of voids less than about three times that of the mean volume of the flow medium passing in one direction through said regenerator.

9. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an out et opening interconnecting said chambers: a heat regenerator comprising a mass of finelv divided filling material comprising elements having a cross-sectional area within the range of about 3 10 cm. to about x10- cm. said mass having a ratio of surface area to volume within the range of about 25 cmfi /cm. to 750 cm. /cm. bein about 0.3 cm. minimum to about 8 cm. maximum in thickness. having a total heat capacity greater than. about three times the h at capacity of the flow medium passing in one direction through said regenerator and having a total volume of voids less than about five t mes that of the mean volume of the flow medium pas ing in one direction through said regenerator.

10. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material comprising elements having a cross sectional area within the range of about 1 10- cm. to about 300x10- cm. said mass having a ratio of surface area to volume within the range of about 100 cmP/cm.

to 200 cmP/cmi, being about 0.8 cm. minimum to about 4 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the flow medium passing in one direction'through said regenerator and having a total volume of voids less than about five "times that of the mean volume of the flow m ments having a cross sectional area within the range of about 1 10 cm. to about 300x10- cm. said mass having a ratio of surface area to volume within the range of about 25 cmfi/cm. to 750 cmF/cmi, being about 0.8 minimum to 1 about 4 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the flow medium passing in one direction through said regenerator and having a total volume of voids less than about three times Be that of the mean volume of the fiow medium passing in one direction through said regenerator.

12. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a 25 cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material comprising elements having a cross sectional area within the range 0 of about 3 10- cm. to about 40 10- cm said mass having a ratio of surface area to volume within the range of about 25 cmF/cm. to 750 cm. /cm. being about 0.3 cm. minimum to about 8 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the fiow medium passing in one direction through said regenerator and having a total volume of voids less than about three times that of the mean volume of the flow medium passing in one direction through said regenerator.

13. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material comprising elements having a cross sectional area within the range of about 3 10 cm. to about 40x10- cm. said mass having a ratio of surface area to volume within the range of about 100 cmP/cm.

to 200 cm. /cm. being about 0.3 cm. minimum to about 8 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the flow medium passing in one direction through said regenerator and having a total volume of voids less than about five times that of the mean volume of the fiow medium passing in one direction through said regenerator.

14. In a hot-gas apparatus operating with a m given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said -chambers; a heat regenerator comprising a mass of finely divided filling material comprising elements having a cross sectional area within the range of about 1 10 cm. to about 300x10 cm. said mass having a ratio of surface area to volume within the range of about cmF/cm.

to 200 cmF/cmfi, being about 0.3 cm. minimum to about 8 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the flow medium passing (5 in one direction through said regenerator and having a total volume of voids less than about three times that of the mean volume of the flow medium passing in one direction through said regenerator.

15. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material comprising elements having a cross sectional area within the range of about 3 10- cm. to about 40 10- cm. said mass having a ratio of surface area to volume within the range or about 25 cmf /cm. to 750 cmF/cmfi, being about 0.8 cm. minimum to about 4 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the flow medium passing in one direction through said regenerator and having a total volume of voids less than about five times that of the mean volume of the fiow medium passing in one direction through said regenerator.

16. In a hot-gas apparatus operating with a given amount of fiow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material, said mass having a ratio of surface area to volume within the range of about 100 cmP/cm. to 200 cmF/cmfi, being about 0.3 cm. minimum to about 8 cm. maximum in thickness, having a total heat capacity greater than about five times the heat capacity of the flow medium passing in one direction through said regenerator, and having a total volume of voids less than about three times that of the mean volume of the fiow medium passing in one direction through said regenerator, said filling material having a space factor of about 6 to about 25.

1'7. In a hot-gas apparatus operating with a given amount of fiow medium and comprising cylinder means defining a hot chamber and a cold chamber, a. housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material, said mass having a ratio of surface area to volume within the range of about 25 cmF/cm. to 750 cmP/cmfi, being about 0.3 cm. minimum to about 8 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the fiow medium passing in one direction through said regenerator and having a total volume of voids less than about five times that of the mean volume of the flow medium passing in one direction through said regenerator, said filling material having a space factor of about 3 to about 60.

18. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material comprising elements having a cross sectional area within the range of about 3X10- cm. to about 40 10- cm 10 said mass having a ratio of surface area to volume within the range of about 100 cmF/cm. to 200 cmfi/cmfi, being about 0.8 cm. minimum to about 4 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the fiow medium passing in one direction through said regenerator, and havinga total volume of voids less than about three times that of the mean volume of the flow medium passing in one direction through said regenerator, said filling material made from a ferro-alloy selected from the group consisting of ferrochromium or ferro-nickel-chromium.

19. In a hot-gas apparatus.operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material comprising elements having a cross-sectional area within the range of about 3 x 10- cm. to about X 10- cm said mass having a ratio of surface area to volume within the range of about cmf /cm. to 200 cmF/cmfi, being about 0.8 cm. minimum to about 4 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the flow medium passing in one direction through said regenerator, and having a total volume of voids less than about three times that of the mean volume of the flow medium passing in one direction through said regenerator,, said filler consisting of a material of low heat conductivity selected from the group consisting of glass, asbestos and cotton fibers.

20. In a hot-gas apparatus operating with a given amount of flow medium and comprising cylinder means defining a hot chamber and a cold chamber, a housing having an inlet opening and an outlet opening interconnecting said chambers; a heat regenerator comprising a mass of finely divided filling material, said mass having a ratio of surface area to volume within the range of about 25 cmE /cm. to 750 cmF/cmfi, being about 0.3 cm. minimum to about 8 cm. maximum in thickness, having a total heat capacity greater than about three times the heat capacity of the fiow medium passing in one direction through said regenerator and having a total volume of voids less than about five times that of the mean volume of the flow medium passing in one direction through said regenerator, said filling material being of uniform distribution, and having a space factor between about 3 and 60, and further arranged so that all of said flow medium passes through said regenerator.

FRI'IS KAREL nu PRE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 389,045 Bair Sept. 4, 1888 1,730,580 Lundgaard Oct. 8, 1929 1,776,162 Martinka Sept. 16, 1930 1,864,724 Forssbald June 28, 1932 

